1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling a VoIP connection of a mobile device and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple UEs, and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (CoMP), uplink (UL) multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Since large amount of features such as multimedia message, video call, conference call, gaming, file sharing, etc., are supported, a voice over internet protocol (VoIP) service has become more and more popular. Different from a traditional voice service (i.e., a voice call) which is a circuit switched (CS) service, the VoIP service is a packet switched (PS) service and is provided via a data connection, wherein the PS service is supported by the UMTS, the LTE system and the LTE-A system. That is, the data connection is established between a user equipment (UE) and a network in a wireless communication system, for transmitting packets generated for the VoIP service. Besides, the data connection is used for transmitting packets generated for a data service which can be a social network application, a weather application, a stock application, or a messenger application. In general, these applications not only generate the packets when being in an active mode (i.e., operated by a user of the UE), but also generate the packets when being in an idle mode (i.e., executed in background). Thus, the UE (or the user of the UE) may disable a data connection function controlling one or more data connections, to avoid the packets being transmitted unexpectedly and a high network fee is caused. This is especially true when the UE is not in a home network of the UE, i.e., the UE is roaming. Since the network fee is extremely high (i.e., the UE is overly charged) when the UE is roaming, the user prefers to disable the data connection function when the UE is going to leave the home network (prepare to roam).
However, since the VoIP service is performed via the data connection, and cannot be performed when the data connection function is disabled. Inconvenience is caused to the user. For example, the user may simply give up using the VoIP service. Alternatively, the user may need to enable the data connection function, perform the VoIP service, and then disable the data connection function, to avoid being charged overly. Therefore, how to solve the inconvenience caused when using the VoIP service without being charged overly (especially when the UE is roaming) is a topic to be discussed